Lithography is used in the fabrication of semiconductor devices. In lithography, a light sensitive material, called a “photoresist”, coats a wafer substrate, such as silicon. The photoresist may be exposed to light reflected from a mask to reproduce an image of the mask. When the wafer and mask are illuminated, the photoresist undergoes chemical reactions and is then developed to produce a replicated pattern of the mask on the wafer.
Extreme Ultraviolet (EUV) lithography is a promising future lithography technique. EUV light may be produced using a small, hot plasma that will efficiently radiate at a desired wavelength, e.g., in a range of approximately 11 nm to 15 nm. The plasma may be created in a vacuum chamber, typically by driving a pulsed electrical discharge through the target material or by focusing a pulsed laser beam onto the target material. The light produced by the plasma is then collected by nearby mirrors and sent downstream to the rest of the lithography tool.
The hot plasma tends to erode materials nearby, e.g., the electrodes in electric-discharge sources. The eroded material may coat the collector optics, resulting in a loss of reflectivity and reducing the amount of light available for lithography. Furthermore, in sources that use a metal vapor, such as tin or lithium, the plasma “fuel” itself may contaminate collector mirror surfaces, further reducing reflectivity.